Compact sources generating ultrashort optical pulses near 1550 nm wavelength range are widely regarded as a key enabling technology in developing future optical networks. Several transmission schemes, such as described by Nakazawa et al. in Electron. Lett., vol. 34, pp. 907-909, 1998 and Boivin et al. in Photon. Technol. Lett., vol. 11, pp. 1319-1321, 1999, have been proposed to increase the system capacity by efficiently using ultrashort pulse generators. Mode-locked fiber lasers, employing rare-earth doped fiber as the lasing material and exploiting various active and passive mode-locking techniques provide an attractive source of ultra-short pulses with tunable repetition rate and tunable laser wavelength. See for example U.S. Pat. No. 5,008,887 to Kafka and U.S. Pat. No. 5,050,183 to Duling.
Passive mode-locking techniques based on saturable absorber are the most promising as it concerns the pulse-width and the simplicity of the laser cavity, as described by Collings et al. in J. Sel. Top. in Quantum Electron., vol. 3, pp. 1065-1075, 1997 and U.S. Pat. No. 6,097,741 to Lin et al. A saturable absorber imposes an intensity dependent nonlinear effect on a light beam incident upon it. An incident radiation of low intensity is absorbed while a high intensity radiation is permitted to pass the absorber with much less attenuation. Thus, when used in a laser cavity the saturable absorber will introduce intensity dependent losses. Because the laser tends to operate with minimum cavity loss per round-trip the longitudinal modes of the lasers are locked together in phase corresponding to high intensity short optical pulses in the time domain. A mode-locking technique which relies upon the use of the nonlinear reflectivity of a semiconductor saturable absorber mirror (SESAM) is attractive because it eliminates the need for critical cavity alignment, it can be designed to operate in a wide spectral range, has ultrafast nonlinear dynamics and relatively large nonlinear reflectivity changes. Ultrashort optical pulses have been produced with this technique using different semiconductor structures and mirror designs. See for example U.S. Pat. No. 5,627,854 to Knox, U.S. Pat. No. 5,237,577 to Keller and Zhang et al., Appl. Phys. B, vol. 70, pp. 59.62, 2000.
In order to achieve gigahertz repetition rate a mode-locked laser has to be operated at a harmonic of the fundamental frequency. It should be noted, however, that due to the long relaxation time of the amplifier, the laser will only saturate in the average power and not individual pulses, as thought by Harvey et al. in Opt. Lett., vol. 18, pp. 107-109, 1993. Therefore the output of such a laser suffers increased pulse-to-pulse instability and supermode competition, thus leading to pulse dropouts and repetition rate instability. Harmonic mode-locking stabilization, a primary condition for telecommunication applications, has been achieved using both active and passive techniques.
Actively mode-locked lasers produce pulses with excellent stability, however pulses are typically much longer than those obtained by passive mode-locked lasers. To further reduce the pulse width, actively mode-locked fiber lasers synchronized to an external clock have been demonstrated employing soliton pulse shortening, as demonstrated by Kafka et al. in Opt. Lett. vol. 14, pp. 1269, 1989 and Carruthers et al. in Opt. Lett., vol. 21, pp. 1927-1929, 1996, or other passive pulse shaping techniques, see Okhotnikov et al., Photon. Tech. Lett., vol. 14, 2002. Further details for producing short duration pulses are disclosed in the technical paper of D. J. Jones et. al, Opt. Lett., vol. 21, pp. 1818, 1996. Moreover, it was shown that in a harmonically active mode-locked fiber ring laser (Thoen et al., Opt. Lett., vol. 25, 948-950, 2000), the amplitude fluctuations can be significantly reduced, thus pulse dropouts are eliminated, owing on optical limiting action of two-photon absorption in semiconductor saturable absorbers. However, an actively mode-locked fiber laser requires advanced modulation devices and driving electronics, resulting in complicated and ultimately expensive laser cavities.
In contrast, Fermann et al., see U.S. Pat. No. 5,414,725 and Okhotnikov et al., see Appl. Phys. B, vol. 72, pp. 381-384, 2001 demonstrated a passive technique to produce ultrashort pulses with stable repetition rates comparable to that of typical mode-locked lasers. The stabilization of the repetition rate was achieved by harmonic partitioning of the laser cavity by a semiconductor saturable absorber which is preferentially bleached when the two pulse streams circulating within the main cavity of length n×L, respective subcavity of length L collide upon the saturable absorber. While this system proved very efficient in stabilizing the repetition rate it has the limitation of a fixed repetition rate with selection of the positioning of the saturable absorber and is only adjustable by physically moving the intra-cavity elements defining the main cavity, respective the ‘harmonic’ sub-cavity.
In another attempt, generation of ultrashort pulses with stable and adjustable repetition rate from passively mode locked fiber lasers was achieved by using a semiconductor saturable absorber with a life-time of the order of 10 ns into a fiber lasers with cavity round-trips of the order of 100 ns, as further disclosed in U.S. Pat. No. 5,701,319. However the pulse jitter was limited to 300 ps and 50 ps for a repetition rate of 20 MHz, respectively 500 MHz. Yet it is believed that such a laser generates a harmonic pulse train with a reduced long-term stability of the repetition rate.
Grudinin et al. suggested, in Electron. Lett., vol. 29, pp. 1860-1861, 1993, that soliton interplay, through long-lived acousto-optic interactions, can stabilize the repetition rate of a passive harmonic mode-locked pulse train. However, this method requires large intracavity powers to achieve sufficient nonlinearities for stable operation. Thus, cavity lengths of 15 m and longer are required resulting in low fundamental repetition rates which in turns leads to environmental instability and calls for pump levels of a few hundred milliwatts in order to achieve repetition rates of hundred MHz. Yet, more recently, Gray et al. postulated that phase effects in semiconductor saturable absorber could lead to pulse repulsion, which provides in turn self-stabilization of the pulse repetition rate, see Opt. Lett., vol. 21, pp. 207-209, 1996. However, pulse self-organization provided by saturable absorber is sensitive to absorber lifetime and uniform pulse distribution was not observed for absorbers with a carrier lifetime of less than 500 ps. Moreover the repulsive forces between pulses, responsible for self-organization, are sensitive to amplitude fluctuations, therefore optical limiting has to take place in order to build up a stable pulse train.
Recently, generation of an equally spaced soliton pulse train from a short-cavity harmonic mode-locked fiber laser employing a passive pulse formation mechanism, i.e. semiconductor saturable absorber, has been achieved by modulating the cavity loss through optical pumping the saturable absorber by a control beam, as described by Banadeo et al. in Opt. Lett., vol. 25, pp. 1421-1423, 2000. The modulation beam, with a wavelength above semiconductor absorber bandgap, was generated by externally modulating a semiconductor laser placed outward the fiber laser cavity. It was shown that the time ordering could be dramatically improved by this method given that the modulation beam has a frequency of a high harmonic of the fundamental repetition rate. The method resulted in 35-dB suppression of the undesired harmonic modes of a 1.244 GHz-pulse train. Thus, it was demonstrated that optical modulation of the saturable absorber has the potential to generate a jitter-free pulse train with controlled repetition rates and reduced complexity. Roth et al. further developed the method by using a directly modulated high-speed laser diode and implementing a simpler design for internally pumping the saturable absorber, see Electron. Lett., vol. 38, pp. 16-17, 2002.
Although the above-described method provides the means to reduce the complexity of the laser cavity, the use of an additional semiconductor laser as pumping sources for providing optical modulation of the saturable absorber, sets a limit in the attempt of achieving cost effective laser designs.